1. Field of the Invention
The present invention relates to an autotensioner which is used to apply appropriate tension to the timing belt of an automotive engine or to a belt for driving auxiliary machinery, for example, an alternator, compressor, etc.
2. Description of the Prior Art
To drive a cam shaft of an OHC or DOHC type engine to rotate synchronously with the crankshaft, a drive mechanism that employs a timing belt 1, such as that shown in FIG. 14, is widely used.
In FIG. 14, reference numeral 2 denotes a driving pulley that is driven to rotate by the crankshaft of an engine, 3 a driven pulley that is secured to an end portion of a cam shaft, and 4 a tension pulley for applying appropriate tension to the timing belt 1.
The tension pulley 4 is, as shown in the enlarged view of FIG. 15, rotatably supported by a portion of a pivoting member 6 that pivots about a fixed shaft 5, the portion being eccentric with respect to the fixed shaft 5. A tension spring 8 is connected at one end thereof to the distal end portion of an arm piece 7 that is secured at its proximal end to the pivoting member 6, thereby applying resilient force to the pivoting member 6 in a direction in which the tension pulley 4 is resiliently pressed against the timing belt 1, and thus maintaining the tension in the timing belt 1 at a constant level irrespective of a change in the size of the timing belt 1 caused by a temperature change, for example, or oscillations of the belt 1 caused by the operation of the engine. This machanism is generally known as autotensioner.
The conventional autotensioner, however, involves the following problems.
When the driving pulley 2 in the arrangement shown in FIGS. 14 and 15 rotates counterclockwise, as shown by an arrow a in FIG. 14, the left half of the timing belt 1 tends to become taut, while the right half tends to become slack.
The autotensioner, which includes the tension pulley 4, is provided at the right half of the timing belt 1, that is, the portion of the belt 1 which tends to become slack. However, when the engine comes to a stop, it is likely to momentarily rotate in the reverse direction. During this moment, the right half of the timing belt 1 tends to become taut.
If the tension pulley 4 directly follows the movement of the timing belt 1 when such a sudden change in tension occurs, a large amount of slack momentarily occurs in the timing belt 1. In an extreme case, the slack in the timing belt 1 causes an undesired shift in the mesh between the belt 1 and the toothed pulleys (driving and driven pulleys 2 and 3), resulting in a difference in the phase of rotation between the engine crankshaft and the cam shaft.
To solve this problem, damper resistance that occurs between the fixed shaft 5 and the pivoting member 6 may be utilized in such a manner that the tension pulley 4 will not immediately follow a sudden change in the tension. In such a case, however, when the tension pulley 4 is rotating in a normal state (i.e., tension variations are small), it may be unable to follow fine oscillations of the timing belt 1. Thus, this arrangement may cause oscillations of the timing belt 1.
Under these circumstances, Japanese Patent Public Disclosure (KOKAI) No. 63-167163 discloses an invention wherein an oil damper mechanism and a roller-type one-way clutch are provided around the fixed shaft 5 so that the tension pulley 4 immediately follows the movement of the belt 1 only when the belt 1 becomes slack.
The disclosed invention suffers, however, from a lack of durability due to the following reasons: it is difficult to lubricate the roller type one-way clutch; fretting corrosion is likely to occur due to the type of structure; and the tension in the belt is supported by a roller.